1. Field of the Invention
The present invention relates to an integrated circuit with a MOS output stage transistor.
2. Description of the Background Art
Integrated circuits are used, for example, in the area of field bus systems. In this case, a bus line of the field bus system is connected, for example, to a drain electrode of the output stage transistor, and a source electrode of the output stage transistor is connected to ground. The bus line is connected to an operating voltage via a pull-up resistor. If all participants in the bus system drive their respective output stage transistor in such a way that it blocks, an operating voltage is present at the bus line. If at least one of the participants switches on its respective output stage transistor, a ground potential is present at the bus line.
Because, the operating voltage of the bus system is typically greater than a supply voltage of the bus participants, the bus line is not connected directly to a bus participant or its control unit, for example, a microprocessor. In this case, the control unit provides only a drive signal at its supply voltage level, which causes the output stage transistor to turn on or off, and applies the drive signal to a gate terminal of the integrated circuit.
To change a switching state of the output stage transistor, a charge transistor and a discharge transistor are typically provided, which supply a charging current or a discharge current to the gate electrode of the output stage transistor for switching or recharging a gate voltage. For this purpose, a charging current source is looped in series with the charge transistor between the supply voltage and the gate electrode, and the discharge transistor in series with a discharge current source between the gate electrode and ground. The current sources during a recharging process supply a substantially constant current, as a result of which more or less constant signal change rates or switching edges result.
The signal courses produced by switching the output stage transistor on the bus line must typically meet certain boundary conditions specified in a bus specification. These include, for example, a signal rise rate or a signal reduction rate, which are also called a slew rate, and switching time durations from a low to a high level or from a high to a low level of the bus signal, which ideally are to be equal in length. At the same time, a time delay between the drive signal provided by the control unit and the associated bus signal is to be as small as possible.
German patent publication DE 41 31 783 C1, which corresponds to U.S. Pat. No. 5,289,051, discloses a generic circuit configuration for driving a MOS power transistor with connected auxiliary current sources to provide an additional charging or discharge current.
A method and a device for driving a power output stage is known from German patent publication DE 198 55 604 C1, which corresponds to U.S. Pat. No. 6,556,407, in which a power transistor is charged or discharged with different charging current intensities.